Specifications

42 Chapter 6
Making Distortion Measurements
Third-Order Intermodulation Distortion
Making Distortion Measurements
Third-Order Intermodulation Distortion
Two-tone, third-order intermodulation distortion is a common test in
communication systems. When two signals are present in a non-linear
system, they can interact and create third-order intermodulation
distortion products that are located close to the original signals. These
distortion products are generated by system components such as
amplifiers and mixers.
For the quick setup TOI measurement example, refer to “Measuring
TOI Distortion with a One-Button Measurement” on page 44.
This procedure tests a device for third-order intermodulation using
markers. Two sources are used, one set to 300 MHz and the other to
301 MHz.
Step 1. Connect the equipment as shown in Figure 6-3. This combination of
signal generators, low pass filters, and directional coupler (used as a
combiner) results in a two-tone source with very low intermodulation
distortion. Although the distortion from this setup may be better than
the specified performance of the analyzer, it is useful for determining
the TOI performance of the source/analyzer combination. After the
performance of the source/analyzer combination has been verified, the
device-under-test (DUT) (for example, an amplifier) would be inserted
between the directional coupler output and the analyzer input.
Figure 6-3 Third-Order Intermodulation Equipment Setup
NOTE The coupler should have a high degree of isolation between the two
input ports so the sources do not intermodulate.
Step 2. Set one source (signal generator) to 300 MHz and the other source to
301 MHz, for a frequency separation of 1 MHz. Set the sources equal in
amplitude as measured by the analyzer (in this example, they are set to
−5dBm).